SECTION NOTES: MUST-KNOW CHEMISTRY
SHAPES OF MOLECULES (VSEPR) AND HYBRIDISATION
1. THE RULE
Electron pairs around the central atom repel each other and spread out as far apart as possible. Lone pairs repel more strongly than bonding pairs, so they squeeze the bond angles:
lone pair–lone pair > lone pair–bond pair > bond pair–bond pair
Each lone pair reduces the bond angle by roughly 2.5°.
2. SHAPES TABLE
| Bond pairs | Lone pairs | Shape | Bond angle | Hybridisation | Examples |
|---|---|---|---|---|---|
| 2 | 0 | Linear | 180° | sp | BeCl2, CO2, C2H2 |
| 3 | 0 | Trigonal planar | 120° | sp2 | BF3, SO3, BCl3 |
| 2 | 1 | Bent (V-shaped) | less than 120° (about 119° in SO2) | sp2 | SO2, O3 |
| 4 | 0 | Tetrahedral | 109.5° | sp3 | CH4, CCl4, NH4+ |
| 3 | 1 | Trigonal pyramidal | 107° | sp3 | NH3, PCl3, H3O+ |
| 2 | 2 | Bent (V-shaped) | 104.5° | sp3 | H2O, H2S, OF2 |
| 5 | 0 | Trigonal bipyramidal | 90° and 120° | sp3d | PCl5 |
| 4 | 1 | See-saw | less than 90° and 120° | sp3d | SF4 |
| 3 | 2 | T-shaped | less than 90° | sp3d | ClF3 |
| 2 | 3 | Linear | 180° | sp3d | XeF2, I3− |
| 6 | 0 | Octahedral | 90° | sp3d2 | SF6 |
| 5 | 1 | Square pyramidal | less than 90° | sp3d2 | BrF5 |
| 4 | 2 | Square planar | 90° | sp3d2 | XeF4 |
3. QUICK COUNTING
Electron pairs on the central atom = (valence electrons of central atom + atoms bonded that each give one electron ± charge) ÷ 2.
A double or triple bond counts as ONE bonding direction.
Hybridisation from the number of electron pairs: 2 → sp, 3 → sp2, 4 → sp3, 5 → sp3d, 6 → sp3d2.
Carbon: single bonds only → sp3; one double bond → sp2; a triple bond or two double bonds → sp.
4. POLAR OR NON-POLAR?
A molecule is non-polar when the bond dipoles cancel by symmetry, even if the bonds themselves are polar.
| Non-polar | Polar |
|---|---|
| CO2, BF3, CH4, CCl4, PCl5, SF6, XeF4 | H2O, NH3, SO2, HCl, CHCl3, PCl3 |